Compressibility effects on hypersonic turbulent channel flow with cold walls

Compressibility effects on velocity and temperature fluctuations in hypersonic turbulence over cold walls are investigated by exploiting a direct numerical simulation database. We found that the compressibility effects are enhanced by the decrease in wall temperature, which is directly reflected by the rapid increase in velocity divergence and turbulent Mach number. Helmholtz decomposition is adopted to evaluate the genuine compressibility effects by splitting the velocity fluctuation into a solenoidal component and a dilatational component. As the wall temperature decreases, the vertical motion is gradually dominated by the dilatational component, while the wall-parallel motions still by the solenoidal component. The dilatational components tend to decrease the skin friction by around 4%-6%. The instantaneous and conditionally averaged field around strong compressive motions further suggests that the dilatational structures behave as traveling-wave packets surrounded by vortex clusters. To improve the strong analogy between temperature and velocity fluctuations, their correlation in spectral space is studied. The results show that they are strongly correlated at the scales of energy-containing motions, but they are mutually independent at small scales. Published under an exclusive license by AIP Publishing.

Automated summary

The effects of compressibility in hypersonic turbulence over cold walls are enhanced as wall temperature decreases, leading to rapid increase in velocity divergence and turbulent Mach number. As wall temperature decreases, dilatational components gradually dominate vertical motion.